The present invention relates to a positive electrode active material for alkaline storage batteries, and also a positive electrode and alkaline storage battery using thereof.
In recent years, the capacity density of a positive electrode for an alkaline storage battery has been significantly increased by improvements of the substrate shape, active material shape, active material composition and additives. At present, a positive electrode having a capacity density of around 600 mAh/cc has been put into practical application.
However, equipments using an alkaline storage battery as a power source demand further improvements of the high-rate discharge characteristic and output.
In order to improve the high-rate discharge characteristic, conventionally, there have been considered a method of improving the current collecting efficiency of the electrode; a method of lowering the resistance of the electrode; and a method of improving the charge/discharge efficiency of the active material.
Moreover, modification of nickel hydroxide has been carried out by replacing Ni in the nickel hydroxide with metal elements other than Ni.
Application to the electrode material of nickel hydroxide in the state of a solid solution containing a small amount of magnesium has been studied because of its high discharge potential. The battery output is significantly improved when discharge potential shifts in a higher direction (noble direction). Further, when the nickel hydroxide in the state of a solid solution containing a small amount of magnesium is used as a positive electrode active material, since the formation of xcex3-NiOOH is limited, the cycle life of the battery is improved.
As the nickel hydroxide in the state of a solid solution containing a small amount of magnesium, the following have been proposed.
(1) Japanese Laid-Open Patent Publication No. Hei 2-109261 (1990) proposes nickel hydroxide in the state of a solid solution containing 1 to 3 wt % of magnesium, wherein the inner pore radius is 30 xc3x85 or less and the total pore volume is 0.05 ml/g or less. The purposes of this proposal are to increase the density of nickel hydroxide and provide an active material having a long life and a high utilization rate.
(2) Japanese Laid-Open Patent Publication No. Hei 5-21064 (1993) proposes nickel hydroxide in the state of a solid solution containing 1 to 7 wt % of magnesium or the like, comprising a mixture of spherical or substantially spherical particles and non-spherical particles. The purpose of this proposal is to increase the nickel hydroxide content in the positive electrode.
(3) Japanese Laid-Open Patent Publication No. Hei 5-41212 (1993) proposes nickel hydroxide in the state of a solid solution containing 1 to 7 wt % of magnesium or the like, wherein a myriad of primary particles of not larger than 0.1 xcexcm gather and the volume of pores with a pore diameter of 30 xc3x85 or more is 20 to 70% of the total void volume. The purpose of this proposal is to facilitate infiltration of an electrolyte into the particles, thereby limiting the formation of xcex3-NiOOH due to uneven distribution of the electrolyte in the particles and improving the utilization rate of the active material at an early stage of charge/discharge.
(4) Japanese Laid-Open Patent Publication No. Hei 5-182662 (1993) proposes nickel hydroxide in the state of a solid solution obtained by replacing a part of Ni with other element, wherein the inner pore volume is 0.14 ml/g or less. As the other element, Zn, Mg, Cd or Ba is used because they do not impair the characteristic of nickel hydroxide as an active material. The purpose of this proposal is to replace a part of Ni in high-density nickel hydroxide with an element such as magnesium so as to form a defect in the crystal lattice of the nickel hydroxide, thereby increasing the degree of freedom of proton transfer and limiting the formation of xcex3-NiOOH.
(5) Japanese Laid-Open Patent Publication No. Hei 5-182663 (1993) proposes nickel hydroxide in the state of a solid solution in which a part of Ni is replaced with Co and other element, and the inner pore volume is 0.14 ml/g. As the other element, Zn, Mg, Cd or Ba is used. The purpose of this proposal is to improve the charge efficiency at high temperatures by replacing a part of Ni in the nickel hydroxide with a predetermined element.
(6) Japanese Laid-Open Patent Publication No. Hei 11-219703 (1999) proposes to form a coating layer comprising a cobalt compound containing sodium, on the surface of nickel hydroxide in the state of a solid solution containing 0.5 to 5 wt % of magnesium and to contain 0.05 to 5.0 wt % of yttrium based on Ni in the nickel hydroxide in a non-sintered type nickel positive electrode. The purpose of this proposal is to improve the charge acceptance.
Here, when a solid solution is formed by incorporating magnesium into nickel hydroxide, since a nickel sulfate or the like is used as a starting material, sulfate ions are absorbed into the crystals of the nickel hydroxide and the crystal structure is likely to be disordered. Besides, when the crystal structure of the nickel hydroxide is disordered, the polarization during high-rate discharge increases, resulting in a problem that the conductivity also decreases considerably. Accordingly, during high-rate discharge, the utilization rate of nickel hydroxide tends to be lower. It would be considered that the increase of polarization during high-rate discharge due to the disordered crystal structure of the nickel hydroxide is caused by a decrease in the degree of freedom of proton transfer.
However, all of the above proposals (1) through (6) aim for an improvement of the cycle life or the charge/discharge efficiency and do not disclose means for solving the problem of the insufficient high-rate discharge characteristic. Therefore, even if batteries are manufactured according to the above proposals, sufficient high-rate discharge characteristics are not obtained.
Moreover, the nickel hydroxide in the state of a solid solution containing magnesium has a problem of lower charge efficiency at high temperatures.
Although the above proposal (5) discloses a technique of improving the charge efficiency at high temperatures, it does not solve the problem of the insufficient high-rate discharge characteristic.
In other words, it is difficult to obtain batteries having sufficient high-rate discharge characteristics and an excellent charge efficiency at high temperatures.
An object of the present invention is to provide a positive electrode active material for producing an alkaline storage battery with a high discharge voltage and an excellent high-rate discharge characteristic, and a positive electrode and an alkaline storage battery using the positive electrode active material.
Another object of the present invention is to provide a positive electrode active material for producing an alkaline storage battery with a high discharge voltage, an excellent high-rate discharge characteristic and excellent charge efficiency at high temperatures, and provide a positive electrode and an alkaline storage battery using the positive electrode active material.
In order to achieve the above objects, the present invention relates to a positive electrode active material for an alkaline storage battery comprising a nickel hydroxide powder, wherein the nickel hydroxide is a solid solution containing magnesium, the magnesium content in the nickel hydroxide is 2 to 7 mol % of all metallic elements contained in the nickel hydroxide, the tap density of the nickel hydroxide is 1.9 g/cm3 or more, the half-width of a peak attributed to (101) face near 2xcex8=37 to 40xc2x0 in a powder X-ray diffraction pattern of the nickel hydroxide by CuKxcex1 ray radiation is 0.7 to 1.2xc2x0, and the sulfate ion content in the nickel hydroxide is 0.5 wt % or less.
Here, in the powder X-ray diffraction pattern of the nickel hydroxide by CuKxcex1 ray radiation, the ratio of intensity B of a peak attributed to (001) face near 2xcex8=18 to 21xc2x0 to intensity A of the peak attributed to (101) face near 2xcex8=37 to 40xc2x0: B/A is preferably 1.1 or more.
It is preferred that the nickel hydroxide further contains at least one element selected from the group consisting of cobalt and manganese.
It is preferred that the content of the at least one element selected from the group consisting of cobalt and manganese in the nickel hydroxide is 0.5 to 3 mol % of all metallic elements in the nickel hydroxide.
It is preferred that the surface of the nickel hydroxide is coated with an oxide of cobalt.
The average valence number of cobalt contained in the oxide of cobalt is preferably larger than 3.
The present invention also relates to a positive electrode for an alkaline storage battery, containing a positive electrode active material of the present invention. With the use of a positive electrode containing an active material of the present invention, it is possible to obtain an alkaline storage battery having an excellent discharge voltage and high-rate discharge characteristic.
It is preferred that the positive electrode of the present invention further contains a powder comprising an oxide of at least one element selected from the group consisting of Y, Yb, Lu, Ti and Ca. The content of the powder comprising the oxide in the positive electrode is preferably 0.5 to 3 parts by weight per 100 parts by weight of the positive electrode active material of the present invention for an alkaline storage battery.
The present invention further relates to an alkaline storage battery comprising a positive electrode of the present invention for an alkaline storage battery; a negative electrode; and an alkaline electrolyte.
Here, the alkaline electrolyte preferably contains sodium hydroxide.
The concentration of the sodium hydroxide in the alkaline electrolyte is preferably 1 to 5 mol/liter.